Emission wavelength dependence on the rISC rate in TADF compounds with large conformational disorder

What is it about?

three phenothiazine–pyrimidine TADF compounds with different molecular geometries were synthesized and comprehensively analysed by time-resolved spectroscopy. All compounds showed pronounced conformational disorder governing their emission properties. TADF efficiency and the rISC rate were altered by introducing heavy atoms due to enlarged spin–orbit coupling. A 150 meV redshift of the emission peak in solid films was observed after oxygen removal for compounds with a low rISC rate, while no shift was seen for compounds with rapid rISC. Such unusual behaviour was caused by the quenching of the long-lived DF bearing the largest 1CT energy even in ambient O2 , resulting in the loss of the high-energy shoulder of TIFL spectra. Only minor quenching of the latest delayed emission and no noticeable shift of the TIFL spectrum were observed for compounds with rapid rISC. Our results showcase the importance of the large rISC rate in the TADF properties. We have shown that the emission energy of TADF compounds in solid films may depend not only on the conjugation length, but also on some unexpected parameters, such as the rISC rate or molecular core rigidity. This should be taken into account while designing novel TADF compounds.

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